1. Field of the Invention
This invention relates to a photolithography process, and more particularly to a method of fabricating a phase shifting mask (PSM).
2. Description of Related Art
As the integration of an integrated circuit (IC) device increases, a photolithography technology with high light resolution is required to achieve precise fabrication of the IC device. A light source with shorter wavelength is proposed as a way of meeting this high light resolution requirement. A krypton fluoride laser is an example of an ultraviolet source with a wavelength of 2480 .ANG. for exposure use. However, although a light source with shorter wavelength can increase the light resolution, it can also cause its resulting depth of focus (DOF) to be insufficient. Another way to increase light resolution is to use a PSM in the photolithography process. The use of PSMs has become a trend, and so manufacturers endeavor to devote more effort to the R&D of PSMs.
The PSM uses a shifter layer formed over a typical photo-mask, in which the shifter layer can invert the phase of a light ray. When the PSM is exposed, the light rays passing through the shifter layer with inverted phase interfere with the other light rays so that the patterns exposed on a semiconductor wafer have a better pattern resolution. The PSM has an advantage in that there is no need of a new light source to increase the pattern resolution through a modification on the typical photo mask, even though the fabrication of the PSM may be complicated.
A half tone PSM (HTPSM) is usually used to form a hole pattern in photolithography technology. Because a positive photoresist is generally used to form the hole pattern mask, conventional HTPSM entails a shifter layer with the hole pattern formed over a transparent substrate. The shifter layer usually has a transmission coefficient of only about 3-10% and can shift the phase of passing light with a shift angle of 180.degree., which means that the phase of the passing light is inverted. Due to the shifter layer, as seen in FIG. 1, the light contrast between the openings 110, 112 and the shifter layer 102 is increased and the pattern resolution is thereby increased.
FIG. 1 is a bottom view of a schematic conventional HTPSM. FIG. 2 is a cross-sectional view of the schematic conventional HTPSM along the line of I--I in FIG. 1. Referring to FIG. 1 and FIG. 2, a conventional HTPSM includes a transparent substrate 100 with a shifter layer 102 on it. The transparent substrate 100 includes a transparent material such as quartz, and the shifter layer 102 includes a material such as MoSi.sub.z O.sub.x N.sub.y, or SiO.sub.x N.sub.y. An opening 110 and an opening 120 are formed on the shifter layer 102 to expose the transparent substrate 100. In a photolithography process, when the light rays pass through the HTPSM, the opening 110 is for forming a hole pattern, and the opening 120 is for forming an alignment mark such as a reticle alignment mark (not shown), which is used for alignment in several subsequent fabrication processes.
FIG. 3 is the light amplitude distribution on an exposed wafer along the line I--I in FIG. 1. As seen in FIG. 2 and FIG. 3, the shifter layer 102 shifts the light phase with a shift angle of 180.degree. and has a transmission coefficient of about 3-10% under an exposure of an i-line ray with a wave length of 365 nm or a deep ultra-violet (UV) ray with a wave length of 248 nm. The positive light amplitude distribution is from the light passing through the openings 110, 120, and the negative light amplitude distribution is from the light passing through the shifter layer 102. A light intensity is obtained by taking the square of the summation of the positive amplitude and the negative amplitude. The light intensity distribution on the wafer (not shown) can be expected to have a better pattern resolution due to a subtraction of the negative amplitude on several critical places. The critical places are located on the edges of the opening 110, 120 on the shifter layer 102 due to the abrupt change of the light phase.
Even though the shifter layer 102 can improve the pattern resolution, it causes a serious issue regarding the question of mask alignment. Conventionally, a red-light laser with a length of 488 nm or a light emission diode (LED) with a wavelength of 633 nm is used to align the photo mask. These two kinds of light have a larger transmission coeffcient of greater than 50% on the shifter layer 102. FIG. 4 is the light amplitude distribution on an exposed wafer along the line I--I in FIG. 1, in which the light source is for the purpose of aligning the photo mask. In FIG. 4, the positive light amplitudes are from the light ray passing through the openings 110 and 112, and the negative light amplitudes are due to the light rays passing through the shifter layer 102. After taking the square of the summation of the positive amplitude and the negative amplitude and automatically amplifying the light amplitude through an auto-gain device, the light contrast (not shown) for the opening 120 is critically poor.
The poor light contrast of the opening 120 results in a difficulty in mask alignment, and even may result in a failure of mask alignment. This is a drawback of the conventional HTPSM.